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Some Studies on the New Zealand Oysters

Ostrea lutaria Hutton, 1873 (Pl. 2)

Ostrea lutaria Hutton, 1873 (Pl. 2)

  • 1873 (non) Ostrea purpurea Hanley. Hutton, p. 84.
  • 1873 Ostrea lutaria Hutton. Hutton, p. 84.
  • 1873 (non) Ostrea virginica Lamarck. Hutton, p. 84.
  • 1880 (non) Ostrea edulis Linnaeus. Hutton, p. 175.
  • 1880 (non) Ostrea discoidea Gould. Hutton, p. 175.
  • 1884 (non) Ostrea edulis purpurea Hanley. Hutton, p. 533.
  • 1884 (non) Ostrea edulis angasi Hutton. Hutton, p. 533.
  • 1884 (non) Ostrea discoidea Gould. Hutton, p. 533.
  • 1904 (non) Ostrea angasi Sowerby. Hutton, p. 93.
  • 1904 (non) Ostrea purpurea Hanley. Hutton, p. 93.
  • 1913 (non) Ostrea angasi Sowerby. Suter, p. 888.
  • 1913 (non) Ostrea hyotis Linnaeus. Suter, p. 889.
  • 1923 (non) Ostrea angasi Sowerby. Oliver, p. 182.
  • 1928 (non) Ostrea sinuata Lamarck. Finlay, p. 264.
  • 1928 Ostrea charlottae Finlay. Finlay, p. 265 (nom. nov. for O. hyotis Suter non Linnaeus).
  • 1957 (non) Ostrea sinuata Lamarck. Powell, p. 78.
  • 1957 Ostrea charlottae Finlay. Powell, p. 78.

Description: Shell shape generally subtriangular in outline ranging from a subcircular form when growing in uncrowded conditions to an elongate form when growing in crowded conditions. Oysters found in shallow beds have thinner shells than those found in deeper beds, the shells being imbricate and frilled when growing recumbent on a shifting substrate such as mud or sand. Right upper valve flat, sometimes domed just under hinge; distally the valve bears dark brown imbricating prismatic scales which are broad and not crowded together. The scales extend to the outer margin of lower valve so that both distal margins meet. Edges of valves not known to be crenulated. Internally valve colour is white or greenish-yellow and there may or may not be a darker brown band on the distal margin of the valve. Ligament triangular, slightly raised from surface of valve and dark brown in colour. Posterior muscle scar is central in position, broadly lunate, anteroventral region extending further forward. Impression of 'catch' muscle at least 50 per cent or more larger in area than impression of 'quick' muscle area.

The left lower valve is the valve by which the oyster is attached, but in adult specimens the attachment is usually lost and the oyster lies free on the substrate. Valve moderately convex, usually smooth but low distinct radiating ribs and prominent imbricate frills may be present. Beaks usually blunt but may sometimes be more acute; very common on lower left valve whereas in right upper valve they tend to be quickly worn away. In Wellington Harbour oysters, colour of left valve is greenish-white whereas in oysters from Foveaux Strait valve is yellow to dark brown. Internally valve is nacreous or dull, greenish-white. Muscle impression may be slightly larger or smaller than that found in right valve. Ligament is winged laterally so that it is triangular, also elevated.

Denticles occasionally present in young specimens and in oysters that have been growing attached to wharf piles in a crowded condition. Denticles vary in number from three to seven and are on one or both sides of right valve only. Pits corresponding to denticles are present on lower left valve.

Palps and gills creamish-white; posterior suture well dorsal. Tentacles of the mantle fold are generally unpigmented. Middle fold has two rows of tentacles; an outer row with tentacles three to four times longer than their basal width and page 8 an inner row of tentacles twice as long as they are broad. There are at least three to five smaller tentacles to each large tentacle. The inner fold has regular tentacles four to six times longer than their basal width and very closely placed. There are fewer and smaller tentacles towards the hinge region. There are 11–13 bars per plica of gill.

Mature oyster larviparous and hermaphroditic, spawning for at least eight months of the year between August and March at temperatures as low as 10.0°C; is probably protandric with regular alternating female and male phases; becomes hermaphroditic after second or third breeding season. Fecundity about 1,000,000; egg size at liberation, 200μ–270μ. Larvae are incubated for about 20 days; are ready to settle when they measure approximately 0.44mm × 0.34mm. The free swimming larva has not yet been identified in plankton collections. Settling larvae exhibit an exploratory phase; spat measuring 5.5mm in length have total complement of gills present.

Type Locality: Pelorus Sound; Catlin River (= Catlins River).

Distribution: Ostrea lutaria Hutton is recorded from Port Pegasus, Stewart Island; Waitangi Beach, Chatham Islands; Foveaux Strait, East Coast and Golden Bay, South Island; Wellington Harbour, Pauatahanui Inlet, Waitarere Beach, Patea, Napier, Firth of Thames, Auckland Harbour and Bay of Islands, North Island.

Vernacular Names: Stewart Island oyster; Foveaux Strait oyster; Bluff oyster; mud-oyster; deep-water oyster; dredge oyster.

Hutton (1800, p. 175, 1884, p. 533, 1904, p. 93) with some doubt referred the Dunedin rock-oyster to O. reniformis Sowerby and gave on each occasion Sowerby's description. The systematic status of the Dunedin rock-oyster was queried by Suter (1913, p. 889) who referred it to O. tatei and another oyster from Auckland Harbour to O. reniformis Sowerby. Oliver (1923, p. 182) recognised the O. reniformis of Suter as a synonym of O. corrugata Hutton non Brocchi and said that the "use of the name O. reniformis shows how persistently names once introduced into a fauna are retained. Hutton used it for the Dunedin rock-oyster but Sowerby's description does not agree with any New Zealand species. The locality from which it was collected is unknown and the name is best rejected as indeterminable". Oliver (1923, p. 182) further recognised the Dunedin rock-oyster as an intertidal variant of the mud-oyster. However, this view has never been accepted by other systematists.

Suter (1913, p. 889) referred the Dunedin rock-oyster to O. tatei, a new name for O. hippopus Tate non Lamarck, an Australian shell present only in the Eocene, not applicable to a recent New Zealand shell and as Finlay (1928, p. 265) has said, the name O. tatei can only be used as a substitute for O. hippopus Tate non Lamarck. However, the description is still valid and Finlay (1928, p. 265) renamed the oyster O. hefferdi (incorrect spelling for Hefford) and selected as his neotype a shell from Dunedin Harbour. Powell (1957) followed Finlay in keeping the Dunedin rock-oyster separate from the mud-oyster as also does this account.